1. Field of the Invention
The present invention relates to a method for bending at least two glass plates as element materials to form a curved laminated glass. More particularly, it relates to a method for bending a laminated glass consisting of at least two glass plates to have them possess three-dimensional curved surfaces.
2. Discussion of Background
A laminated glass prepared by laminating two sheets of glass between which an intermediate film such as a polyvinyl butyral film is interposed, followed by bonding them, is widely used for a windshield glass or a rear window glass for automobiles and other vehicles because it is highly safe. Generally, a laminated glass in a curved form is used for the windshield glass for the automobiles from the viewpoint of design of the automobiles.
There have been such problem that when the curved laminated glass is to be prepared and if the radius of curvatures at the mating surface of two glass plates to be laminated do not entirely or partially coincide with each other, either or both the glass plates may be broken at a compressing step; undesired air bubbles may be produced between the mating surfaces for long time use, or the mating surfaces may be separated. Accordingly, when the laminated glass having a curved surface is manufactured, it is necessary to coincide the radius of curvature of each of the two glass plates. From this viewpoint, it is desirable that two overlapped glass plates are simultaneously subjected to a bending operation. When such operation is carried out, it is necessary to prevent the two glass plates from being in contact with each other. For this purpose, powder of a material such as sodium bicarbonate, sellaite and so on is generally provided between the mating surfaces.
When glass plates are bent together into a shallow-bent shape wherein the radius of curvature R of the curved portion is 20,000 mm or greater, the shallow-bent portion can be easily obtained, as originally designed, by using a deadweight bending method. However, when the laminated glass is deeply bent wherein the radius of curvature R of the glass plate is 20,000 mm or smaller; especially the glass is subjected to a deep-bending method to have a three-dimensional curved surface wherein the radius is 10,000 mm or smaller, it is necessary to elevate temperature at the surface of the glass plate to be 630.degree. C. or higher. If two glass plates are subjected to the deep-bending method, the powder such as sodium bicarbonate, sellaite and so on provided between the two glass plates or cullet may bit in the mating surface to thereby cause optical distortion to a glass product.
On the other hand, although the technique that glass plates are bent by pressing them has been well-known, it has been applied mainly to manufacture curved tempered glass plates.
A technique that a laminated glass is bent by using a pressing machine is disclosed, for instance, in Japanese Unexamined Patent Publication Nos. 120524/1981 and 129214/1978.
Of these prior art techniques, Japanese Unexamined Patent Publication No. 120524/1981 discloses that a glass plate softened by heating is pressed by upper and lower pressing molds to be bent in a predetermined shape. In this method, a predetermined final shape can not be obtained with high accuracy, and the construction of the pressing machine becomes complicated.
Japanese Unexamined Patent Publication No. 129214/1978 discloses that a glass plate is bent by controlling the temperature of a heating source which is disposed above the glass plate. In this method, the surface temperature of the glass plate becomes high with the result that there causes reduction in optical characteristics which is undesired for a laminated glass.